The present invention relates to a fixture for making a gear for a gear type misalignment coupling wherein the contact surfaces is crowned to minimize interference between mating teeeth during operation.
Gear type misalignment couplings having crowned teeth are used in normal service to transmit mechanical power from one rotating shaft to another where such shafts are not in exact alignment. This type of coupling often comprises a cylindrical hub mounted on one of the shafts, the hub having external teeth formed on its outer peripheral surface which are in meshing engagement with mating internal teeth on the interior surface of a sleeve that is connected to the other of the misaligned shafts by means of another, similar cylidrical hub.
The sleeve is rotatable about its axis while the hubs and their shafts are rotatable about their own axes but may be misaligned with the axis of the sleeve, either angularly or in parallel offset relationship. The external teeth on the hub may not only slide axially with respect to the sleeve teeth but also roll on the internal sleeve teeth in a manner determined by the curvature of the crowned teeth and the angle of misalignment to which the mesh is subjected. Moreover, coupling hub teeth engage the sleeve teeth along a complex pattern as a given hub tooth proceeds through its orbit of operation and this complexity increases as the angle of misalignment increases. There may be apparent positions of abnormal stress on each tooth of a hub even when finished as described in U.S. Pat. No. 3,125,838 to Crankshaw et al.
Such couplings may be quite large when they are used on ships for transmitting power from the drive shaft to the propeller shaft at relatively low speeds. Smaller couplings may also be used where misalignment angles and/or speeds are high. These couplings must be able to transmit high torques between the shafts and must also be quiet in operation.
It is desirable to have as large a combined area of tooth contact as possible at any operating misalignement angle to transmit torque between the teeth of a hub and a sleeve so as to reduce contact stress to a minimum. This is especially difficult in a gear coupling when the shafts connected thereto become misaligned laterally or angularly. At least one of the members of the coupling must therefore be so constructed that there is bearing and rocking tooth contact for the transmission of torque between a maximum number of the teeth of the coupling, when the shafts are misaligned, to reduce backlash and minimize heavy stresses on individual teeth. This arrangement permits the coupling to operate at a greater degree of misalignment of the shafts, it eliminates vibration and noise, and it permits the use of higher speeds and torque.
Gears have, over the years, been manufactured by one of the following methods:
1. Milling: This is done on a conventional milling machine using a cutter shaped in accordance with the space to be produced between teeth. One tooth is cut at a time, and the work is indexed for each successive tooth. For spur gear teeth the axis of the work is always at 90 degrees to the axis of the cutter and the work does not rotate except while being indexed between cuts.
2. Hobbing: In the hobbing process the hob looks not unlike a threaded member, or worm wheel, with gashes cut parallel to the shaft axis to produce the necessary cutting edges. Both hob and work rotate simultaneously in fixed angular speed relationship. For straight spur teeth the axis of rotation of the hob is tilted appropriately with the axis of the work to compensate for the axial pitch of the hob teeth. The axis of the work is maintained parallel to the direction of motion of the hob.
3. Shaping: In the shaping process the cutter is similar to a section of a small pinion, and simultaneously rotates slowly on its axis and reciprocates rapidly in a direction parallel to its axis. The work rotates in fixed speed relationship with the rotation of the cutter. The cutter is first fed into the work radially to its proper depth, and the teeth are generated as the cutter and work revolve.
4. Grinding: Wherein a grinding wheel is substituted for the cutter in one or more of the above processes.
Note that in all of the above methods the final tooth is straight sided in an axial direction. Detailed descriptions of these processes are contained in many books on gearing such as "Gear Cutting Practice": by Colvin & Stanley, McGraw-Hill Book Company, Inc. A Treatise on Gear design for Transmissions was published by Botstiber and Kingston in "Machine Design", Dec. 1952, pp. 130 to 160.